Modern gas turbine engines operate in high temperature environment in excess of 2000.degree. F. in which hot gases are expanded across rows of turbine blades. Outer air seal or shroud segments circumscribe the turbine blades to minimize leakage of the gases over the tips of the blades. The use of thermal barrier coatings on gas turbine blades and surfaces such as such as shroud segments have been found to have several advantages. Through the use of thermal barrier coatings higher operating efficiency can be obtained because less cooling air is required to maintain blade or shroud temperatures. In addition, component life is extended since the rate of change of metal temperature is reduced by the insulating effect of the thermal barrier.
Zirconia based thermal barrier coatings, because of their low thermal conductivity, are added to the surface of metal components to insulate them from the hot gas stream. Stabilized zirconia was developed and used as a thermal barrier coating for turbine and shroud components. Coatings such as CaO stabilized zirconia, MgO stabilized zirconia and Y.sub.2 O.sub.3 stabilized ziconia have been tested with Y.sub.2 O.sub.3 partially stabilized zirconia providing the best results.
U.S. Pat. No. 4,377,371 discloses an improved thermal shock resistance of a ceramic layer in which benign cracks are deliberately introduced to a plasma-sprayed ceramic layer. The benign cracks are generated by scanning a laser beam over the plasma-sprayed ceramic surface where the ceramic material immediately beneath the beam melts to produce a thin fused layer. Shrinkage accompanying cooling and solidification of the fused layer produces a network of microcracks in the fused layer that resists the formation and growth of a catastrophic crack during thermal shock exposure. Another method disclosed for introducing fine cracks on the surface of a ceramic coating is to quench the surface of the ceramic while it is hot with an ethanol saturated paper pad.
An article published by the AIAA/SAE/ASME 16th Joint Propulsion Conference, June 30-July 2, 1980, Development of Improved-Durability Plasma Sprayed Ceramic Coatings for Gas Turbine Engines by I. E. Summer et al, discloses that the durability of plasma sprayed ceramic coatings subjected to cyclic thermal environment has been improved substantially by improving the strain tolerance of a ceramic structure and also by controlling the substrate temperature during the application of the coating. It further states that the improved strain tolerance was achieved by using ceramic structures with increased porosity, microcracking or segmentation.
In an article published by J. Vac. Sci. Technol. A3 (6) November/December 1985 titled Experience with MCrAl and Thermal Barrier Coatings Produced Via Inert Gas Shrouded Plasma Deposition, by T. A. Taylor et all, discloses the depositing of a ceramic oxide coating of ZrO.sub.2 -7 wt % Y.sub.2 O.sub.3 onto a coated substrate. The ceramic oxide coating is a thermal barrier coating which has intentionally imparted microcracks having an average spacing of about 15 microns and which are staggered from layer to layer of the coating.
It is an object of the present invention to provide a thermal barrier coating for components intended to be used in cyclic thermal environments in which the thermal barrier coating has deliberately produced macrocracks homogeneous dispersed throughout the coating to improve its thermal fatigue resistance.
It is another object of the present invention to provide a thermal barrier coating for components of turbine engines in which the coating is composed of zirconia partially stabilized by yttria and in which the coating has a density greater than about 88% of theoretical.
It is another object of the present invention to provide a thermal barrier top coating over a bond coating of an alloy containing chrominum, aluminum, yttrium with a metal selected from the group consisting of nickel, cobalt and iron.
It is another object of the present invention to provide a thermal barrier coating for gas turbine blades, vanes and seal surfaces exposed in the hot section of gas turbine engines.
It is another object of the present invention to provide a process for producing a thermal barrier coating having good thermal fatigue resistance.